Usually, in a steel strip producing apparatus, a cold rolling mill and a continuous annealing furnace are located in separate process lines from each other. That is, the cold rolling procedure for a steel strip is carried out independently from the continuous annealing procedure for the steel strip.
It is known that it is possible to sequentially carry out the cold rolling procedure and the continuous annealing procedure for the steel strip in one process line by placing a strip accmulator, for example, a strand looper or loop car, between the cold rolling mill and the continuous annealing furnace, so as to synchronize the cold rolling speed with the continuous annealing speed. It is known that the maximum speed of this type of continuous annealing furnace is about 500 m/min. When an economical accumulating capacity of the intermediate strip accumulator is about 1000 m at maximum, the cold rolling mills should be operated at a speed which is at the highest about 600 to about 700 m/min. That is, in order to connect the cold rolling process line with the continuous annealing process line through an intermediate strip accumulator so as to provide one process line, it is necessary that the cold rolling mills can be operated at the above-mentioned speed. The cold rolling process line may comprise 6 tandem high speed cold rolling mills or 4 tandem 6 high cold rolling mills and, optionally, a cold rolling drawing mill. Also, it is possible to synchronize a descaling procedure with the cold rolling procedure by placing a strip accumulator between a cold rolling mill and a descaling apparatus such as pickling apparatus, located upstream of the entrance of the cold rolling mill.
However, the continuous cold-rolling and annealing process line has not yet been realized, because stopping of the cold rolling part directly causes the continuous annealing part to be stopped and, therefore, the working efficiency of the continuous cold rolling and annealing process line significantly decreases.
The continuous annealing furnace is provided with a heating zone having a large heat capacity. Therefore, when the operation of the continuous annealing furnace is interrupted, it is very difficult to rapidly discharge the remaining heat from the heating zone, even if a heating means such as burner, is stopped. Accordingly, the steel strip located in the heating zone of the continuous annealing furnace is overheated by the remaining heat of the heating zone. This overheating sometimes causes the steel strip to be ruptured inside the continuous annealing furnace due to a heat buckling or drawing phenomenon. When the steel strip is ruptured inside in the furnace, it is necessary to open and cool the furnace, to remove the ruptured steel strip and, then, to charge a steel strip into the furnace. The above-mentioned procedures cause the continuous process line to be stopped over a long period of time and, therefore, the working efficiency of the process line is substantially decreased.
Generally, it is unavoidable that the cold rolling procedure is frequently interrupted due to the changing operation, adjusting operation and/or cleaning operation of the rolls. If the interruption of the cold rolling procedure always causes the interruption of the continuous annealing procedure, the working efficiency of the process line will decrease in a range of from about 10 to about 20%. In this case, it is practically impossible to continuously connect the cold rolling line with the continuous annealing line.
Under the above-mentioned circumstances, it is strongly desired by the steel-making industry to provide a continuous cold rolling and annealing process line which is free from the above-mentioned disadvantages of the conventional continuous lines.